In the technique known as laparoscopic surgery, a surgeon gains access to an abdomen or other body cavity through a small incision, typically having length on the order of one centimeter. A laparoscope is inserted through the incision into the body cavity.
The laparoscope has an open channel extending through it (typically 30 to 40 centimeters in length, with a diameter in the range 5 to 7 millimeters). The surgeon can insert a surgical instrument (such as the beam of a laser) through the channel, and gas (such as carbon dioxide) can also be introduced through the channel (for insufflation). A conventional laparoscope also includes a train of viewing optics (typically having diameter in the range 2 to 5 millimeters) to facilitate imaging of the body cavity.
In a conventional technique for delivering a laser beam through a laparoscope (or another type of endoscope), a direct coupler introduces the beam into the input end of the laparoscope (or endoscope) channel. The direct coupler (which is essentially a long focal length lens) receives the beam from the end of an articulated arm assembly, and directs the beam into the laparoscope (or endoscope). The beam is introduced into the articulated arm assembly by a laser (typically a CO.sub.2 laser which emits radiation having a wavelength of 10.6 microns). An apparatus employing such a direct coupler is described in U.S. Pat. No. 4,917,083, issued Apr. 17, 1990, with reference to FIG. 3.
However, conventional systems using direct couplers have a number of serious limitations and disadvantages. One such disadvantage is that the beam often strikes the walls of the laparoscope (or endoscope) channel as a result of angular misalignment of the beam at the channel entrance. This often results in a scattered and ineffective output beam at the channel's distal end, and in undesirable heating of the channel walls.
Another disadvantage resulting from use of direct couplers in conventional systems is due to the fact that in such systems the F number of the beam cannot be smaller than the ratio L/w, where L is the length of the channel and w is the diameter of the channel. Since the beam's spot size at the channel's distal end is proportional to the F number, a small spot cannot be produced at the distal end when the beam is simply focused into the channel by a direct coupler. This problem is enhanced when gas (such as carbon dioxide gas) flows through the channel, since the gas absorbs power from the beam, thereby reducing the output beam power at the channel's distal end, and forming a thermal lens (which in turn enlarges the beam's spot size at the channel's distal end).
Waveguides have been employed in some conventional laparoscopes (and endoscopes) to deliver a laser beam into a body cavity. A waveguide is a small diameter tube whose walls are highly reflective to laser radiation that strikes the walls while propagating down the tube. Examples of waveguides include optical fibers (filled with solid transparent material) and hollow waveguides (sometimes referred to as "air fibers" or "air core waveguides"). U.S. Pat. No. 4,805,987, issued Feb. 21, 1989, discloses a flexible, hollow waveguide of a type which could conceivably be employed in a laparoscope or an endoscope.
In the endoscope described in above-cited U.S. Pat. No. 4,917,083, a waveguide receives laser radiation focused by a direct coupler.
Use of a waveguide to deliver a laser beam has the serious disadvantage that a waveguide tends to degrade the beam quality (defined as the ratio of the F number of the beam to the spot size at focus). The result is that the spot size at focus is maintained over a much shorter depth of focus than can be achieved by propagating a precisely oriented beam parallel to the central axis of a channel (without reflection from the channel side walls). This short depth of focus limits the beam's usefulness to a surgeon.
While waveguides can in principle deliver a beam with little loss of beam quality, in practical applications this desired result is very difficult to achieve due to the practical limitations of mechanical and optical tolerances. Furthermore, when gas flows through a hollow waveguide, the same problems described above with reference to direct couplers will result, due to absorption of beam power by the gas flowing within the waveguide. Of course, if a solid waveguide is employed (rather than a hollow waveguide), gas cannot flow through the waveguide at all, which additionally limits the utility of the laparoscope (or endoscope) to the surgeon.
Throughout the remaining portion of the specification, including in the claims, the term "endoscope" will be used in a broad sense to denote laparoscopes and other types of endoscopes.
Until the present invention, it was not known how to eliminate the described disadvantages and limitations of conventional apparatus for coupling a laser beam into, and delivering the beam through, an endoscope channel.